Mechanical devices, such as an injection molding machine, often include two or more drive shafts, and use motors that correspond one-to-one to the two or more drive shafts to drive a single mechanically-controlled device. In such a mechanical device that includes the two or more drive shafts, that is, the multiple drive shafts, one of the multiple drive shafts serves as a master shaft while the other shafts serve as slave shafts. Since all the drive shafts drive the same controlled device, the master shaft and the slave shafts are required to synchronize their speeds and positions.
A technique for achieving the synchronized speed operation between the master shaft and the slave shafts includes a method of providing an identical command value to the master shaft and the slave shafts. Unfortunately, even when the identical command value is provided to the master shaft and the slave shafts, a difference in the rotational speed or the position between the master shaft and the slave shafts may occur due to, for example, the variation in characteristics between the motors corresponding to the respective drive shafts, or a disturbance torque. If such a difference in the rotational speed or the position between the drive shafts occurs, the synchronous control cannot be achieved. Patent Literature 1 discloses an example of the technique for achieving the synchronous control in spite of the difference as described above.
Patent Literature 1 discloses a synchronous control device that detects a workpiece conveyance speed that is a rotational speed of a master shaft, and a rotational speed of a seam electrode that is a rotational speed of a slave shaft. On the basis of the difference between the workpiece conveyance speed and the rotational speed of the seam electrode, the rotational speed of the seam electrode is brought into synchronization with the workpiece conveyance speed. The synchronous control device of Patent Literature 1, which uses the difference between the actually detected speeds to execute the synchronous control, achieves the synchronization even in the presence of the variation in the motor characteristics between the master shaft and the slave shaft, or the disturbance torque.